The invention is in the field of prostheses and is more particularly directed to a rotating thumb prosthesis for replacement of the natural carpal joint.
It is well-known to apply surgical methods, first of all implanting joint prostheses for diminishing pains caused to patients by rheumatic inflammation. Finger joint reconstructions entailing prosthesis are primarily carried out on the carpo-metacarpal joint of the thumb.
Prior known prostheses for carpo-metacarpal joint replacement generally consist of two components. These two components of the artificial joint are the trapezine (proximal) component and the metacarpal (distal) component. The trapezine component of the prosthesis is made in most cases of a biocompatible plastic material which is fixed to the trapezial bone (trapezium) by asteointegration or bone cement. The metacarpal component is generally fixed in the first metacarpal bone by a metal stem.
A problem with these conventional prostheses is that the anchoring element supporting the actual prosthesis mechanism loosens and/or is damaged by the load during natural use. On the other hand, the movement with such prostheses is rather limited with respect to the natural joint motion.
In order to eliminate these problems it has been suggested to insert an elastic element made of biocompatible material between the trapezial component and the metacarpal component. Such type of prostheses is disclosed in the FR-PS 2 679 440. The prosthesis disclosed in this reference is provided with an intermediate element in the form of a piston which can freely reciprocate in a hole within metacarpal component. The piston like element is supported in the trapezial component of the prosthesis by a head which is fitted in a hemispherical cup arranged in the trapezial component of the prosthesis. The cup is made of a biocompatible material and is fixed to the trapezial component by bone cement.
The suggested prosthesis, however, has not eliminated the risk of loosening in the trapezium and the original freedom of the motion of the natural joint has not been reached. Furthermore, a new inconvenient effect has been brought with the new construction: the piston arranged in the hole of the metacarpal component makes a "pumping movement" due to the load on the head of the piston. This pumping effect produces different variations of pressure in the hole which cause pain during movement of the joint. Another consequence of the "pumping" is that detrimental dynamic effects are generated which act against the free reciprocating motion of the piston in the hole. Furthermore, there are necessarily dead spaces at the ends of the stroke of the piston and these dead spaces tend to collect organic materials which could decompose there and produce infections with serious consequences.
It is therefore the principal object of this invention to provide a prosthesis for eliminating these inconveniences.